As well as Magnetic Resonance tomography (MR), Positron Emission Tomography (PET) is becoming increasingly widely used in medical diagnosis. While MR involves an imaging method for representing structures and sectional images within the body, PET makes possible a visualization and quantification of metabolism activities in-vivo.
PET uses the particular properties of positron emitters and positron annihilation in order to determine quantitatively the function of organs or cell areas. In such cases the appropriate radiopharmaceuticals which are marked with radio nuclides are administered to the patient. As they decay, the radio nuclides emit positrons which after a short distance interact with an electron, which causes what is referred to as an annihilation to occur. During this process two Gamma quanta occur which fly off in opposite directions (displaced by 180°). The Gamma quanta are detected by two opposite PET detector modules within a specific time window (coincidence measurement), by which the location of the annihilation is determined to a position on the connecting line between these two detector modules.
For verification the detector module must generally cover a large part of the gantry arc length for PET. It is divided up into detector elements with sides of a few millimeters in length in each case. On detection of a Gamma quantum each detector element generates an event recording which specifies the time as a well as the verification location, i.e. the corresponding detector element. This information is transferred to a fast logic and compared. If two events coincide in a maximum time interval then it is assumed that a Gamma decay process is occurring on the connecting line between the two corresponding detector elements. The PET image is reconstructed with a tomography algorithm, i.e. what is referred to as back projection.
The combination of PET with other tomographic methods, especially computer tomography (CT) is known. Combined PET-CT devices typically allow the deficient local resolution of PET systems to be compensated for. At the same time CT offers a presentation of the anatomy of the patient so that, on superimposition of the CT and PET data, it can be established precisely where in the body the PET activity has taken place. With combined PET-CT devices a PET device and a CT device are typically arranged behind one another so that within an examination the patient can be moved seamlessly from one device into the other one. The two measurements can then take place directly consecutively.
A combination of a PET device with an MR device is advantageous since MR gives a higher soft tissue contrast by comparison with CT. Combined MR-PET systems are already known in which the PET detectors are arranged within an opening defined by the MR magnets with gradient system and excitation coil. In such cases they are positioned next to the excitation coil so that the examination volumes of the MR and PET system do not coincide but are offset in the Z direction. Consequently, like the PET-CT system, PET and MR data cannot be measured simultaneously.
In such cases it is especially to be preferred that the PET device be arranged within the MR device and that the two examination volumes are superimposed. In this case both morphological MR data and also PET data can be determined within one measurement run. As well as the effect of time-saving the two image data records can be shown superimposed in a simple manner so that a diagnosis is simplified for the doctor.
For integration of the PET and MR device it is necessary to arrange the PET detectors within the MR device, so that the imaging volumes lie isocentrically. For example the PET detectors can be arranged on a support structure (support bar, gantry) within the MR device. This can for example be 60 detectors in a ring-shaped arrangement on the support bar. For each of the detectors, which can also be combined into detector blocks, a cooling connection and electrical supply leads are required. These are likewise to be arranged in the MR device. In addition a number of signal processing units is required which are likewise arranged in the MR device. These are connected via the electrical leads to the detectors and are used for signal processing.
In the event of the combination of MR and PET in a combined system an attenuation of the Gamma quanta occurs because of everything which lies between the point of origination of the respective Gamma quanta and the PET detector. In the reconstruction of PET images this attenuation must be taken into account in order to prevent image artifacts. Between the point of origination of the Gamma quant in the body of the patient and the verifying PET detector lies on the one hand patient tissue, generally air and a part of the MR-PET system itself, for example cladding of the patient opening or a patient bed. The attenuation values of the components or accessory parts to be taken into account are combined into attenuation mapsμ. In such cases and attenuation map contains attenuation values for each volume element (voxel) of the investigated volume. Thus for example an attenuation map can be created for the patient table. The same typically applies to local coils applied to the patient for MR examinations. To create the attenuation map it is necessary to determine and to collect together the attenuation values. The values can typically be determined by CT imaging or by a PET transmission measurement of the respective component. These types of attenuation maps can be measured once since the attenuation values do not change over the lifetime of the respective component. For attenuation correction large differences in the attenuation between the different tissue, above all soft parts and bones, are primarily of significance.
With PET-CT systems it is known that an attenuation map can be calculated from CT images using the x-ray absorption coefficients and used for the attenuation correction of PET data. This can also be employed in the measurement of attenuation values of the components. With PET systems a direct determination of the attenuation map from the actual measurement data is not possible. Measurements must thus be made in test measurements with homogeneous PET phantoms, so that the intensity of the Gamma quanta arising is known. Alternately the use of x-ray sources with PET systems is known which are moved around the patient. By detecting the radiation of these radiation sources the attenuation is determined, but this is time-consuming.
With MR-PET systems it is desirable to be able to determine the attenuation directly from MR data sets. Such methods are already known.
Thus a method is known from DE 10 2004 043 889 A1 for creating a nuclear medical image. The image is produced from a data set comprising both data of a magnetic resonance examination and also of a PET measurement. A reference MR data set of the area for which the image is to be recorded of a reference patient with an associated correction data set is provided. A transformation which maps the reference MR data set to the MR image is created and applied to the correction data set for creating a transformed correction data set which is registered with the nuclear medical data set. This involves an Atlas-based method for determining the attenuation values which, with the assistance of the measured MR image, are transferred to the PET data set and used for attenuation correction.
A method is known from DE 10 2006 033 383 A1 for determining an attenuation map for a living being. Attenuation values are able to be predicted on the basis of an MR data set by means of a trained algorithm.
In a further known method specific MR sequences are used in order for example to make bones or plastic parts and coils visible. After a segmentation and registration with PET data, attenuation values can be assigned to the corresponding regions. It is likewise known that the spatial position of accessory parts such as local coils through example can be established by markings and assigned attenuation values on the basis of a database.
The known methods operate relatively satisfactorily as such. However each of the known methods is restricted and thus not suitable for complete definition of attenuation maps. Thus for example either the attenuation values of bones or of local coils can be determined with the known methods.